1. Field of the Invention
The present invention relates to an absolute-type encoder, especially to an absolute-type encoder exploiting the architecture of incremental-type encoder.
2. Description of Prior Art
The conventional AC servo motor generally comprises an optical encoder to sense angle information of a rotor; this angle information can be used to determine a stator driving current. Therefore, the speed of the AC servo motor can be precisely controlled
FIG. 1 shows the schematic diagram of a prior art AC servo motor. The angular position of rotor in a motor 10 is detected by an optical encoder 12 and processed by a signal processing unit 20 to obtain angular information. The angular information is processed by a speed estimation unit 14 to obtain an estimated motor rotational speed. A speed controller 30 receives the estimated motor rotational speed and a speed command to control a controller module 32 and an IGBT module 34 in order to generate a motor speed control signal. The motor speed control signal can be used to precisely control the rotational speed of the motor 10.
More particularly, in the servo motor, the position sensor attached to the motor axis is the optical encoder 12. The position precision of the servo motor depends on the resolution of the optical encoder, where the optical encoder 12 can be classified to incremental-type encoder and absolute-type encoder.
The incremental-type encoder can provide information relative to previous position, and the absolute position of its encoder wheel cannot be known after power failure unless the position is reset. Therefore, the incremental-type encoder can not know the absolute position of its encoder wheel after power is just regained after power failure. On the contrary, the absolute-type encoder can always know the absolute position of the output axis without bothering by power failure. No reset operation is necessary after power on from power failure and the operation is simplified.
FIG. 2 shows the schematic view of an optical encoder. The light from a light source 260 reaches a light sensor 240 after passing a rotating wheel 200 and a fixed mask 220. The signal received by the light sensor 240 is varied according to the position change of the rotating wheel 200. Therefore, the position change of the rotating wheel 200 can be known by detecting the signal intensity of the light sensor 240.
FIG. 3 shows a schematic diagram of an absolute-type encoder wheel 300, which is a 6 bit encoder wheel. The absolute-type encoder wheel 300 comprises a round wheel body 302 and a plurality of gratings 304. The gratings 304 include a first grating 304A at innermost orbit and occupying ½ circumference, two second gratings 304B at second innermost orbit and each occupying ¼ circumference, third gratings 304C, fourth gratings 304D, fifth gratings 304E and 32 sixth grating 304F at outermost orbit and each occupying 1/64 circumference. Intensity-changing signal can be obtained along the radial direction and position resolution of 26=64 can be achieved along circumference direction. However, one more orbit is needed when one bit resolution is to be enhanced in the absolute-type encoder wheel 300 shown in FIG. 3. The absolute-type encoder wheel 300 occupies more space when resolution is more demanding. The absolute-type encoder wheel 300 shown in FIG. 3 has limited resolution when its size is restricted.
FIG. 4A shows a schematic diagram of an encoder wheel 400 for an incremental-type optical encoder, which comprises a round wheel body 402 and a plurality of gratings. The gratings include main grating 404A, first sub grating 404B and second sub grating 404C, where the first sub grating 404B and the second sub grating 404C are arranged on two opposite sides of the main grating 404A. FIG. 4B shows the mask 420 associated with the encoder wheel 400, which comprises four rows of gratings 420A.
FIG. 4C shows the light sensor device 440 associated with the encoder wheel 400, which comprises main light sensor units 442A, 444A, 442B, 444B (labeled as A+/B+/A−/B−) corresponding to the main grating 404A. When the encoder wheel 400 rotates, the main light sensor units 442A, 444A, 442B, 444B (labeled as A+/B+/A−/B−) produces four sinusoid-like signals. The four sinusoid-like signals have phases of 0/90/180/270 degrees. The A+/A− signals with 180 degree phase difference are subjected to subtraction operation to obtain a sine signal A without common mode noise. The B+/B− signals with 180 degree phase difference are subjected to subtraction operation to obtain a cosine signal B without common mode noise. The sine signal A and cosine signal B with 90 degree phase difference can be used to judge forward or backward rotation.
The incremental-type optical encoder can obtain incremental position information based on the sine signal A and cosine signal B. To obtain absolute position information, origin light sensor unit 446A, 446B (Z+/Z−) are additionally provided. However, after power on from power failure, an origin mark on the incremental encoder should be sensed by the origin sensor unit to obtain the absolute position information. This process is time consuming and not suitable for application demanding no return to the origin mark.